The laser printer is based on the principle of electrophotography. In electrophotography, a latent image is generated from electric charges by imaging, i.e., exposing a photoconductor with the optical image of an original document, and then using this latent image subsequently to apply a toner selectively (developing) and to produce an image (copy) of the original on paper, for example. A distinction is made between direct and indirect electrophotography as well as between wet and dry electrophotography. The wet methods, also known as liquid toner methods, use a suspension of the toner in an aliphatic solvent having a low dielectric constant and as the developer, whereas the dry method uses a toner powder. Using a bundled laser beam and a rotating mirror, an image of the desired page is recorded on the photosensitive drum. The drum has a negative charge at first, so the charge is cancelled at locations where the laser beam strikes. The shape of the discharged areas on the drum corresponds to that in the subsequent printout. The toner is applied to the drum via a roller using negatively charged toner which adheres to the discharged areas on the drum.
In the dry toner method, the paper is then guided over the drum, barely touching the drum in passing over it. A potential field is built up behind the paper. The toner is transferred to the paper, where it is initially in loose form. The toner is then fused under pressure with the help of a hot roller. The drum is discharged and excess toner is removed from it.
In the liquid toner method, the toner suspension is first transferred to a heated rubber cylinder or belt on which the liquid carrier evaporates and the toner is plastified. The toner image is then transferred from this intermediate drum to the recording sheet.
The images created with the help of a laser printer should achieve a quality comparable to that of a photograph. This includes properties such as gloss, stiffness and opacity, a high resolution and definition as well as a good light fastness.
Multilayer coated papers having an “open” porous surface on both sides are described in the publication by HP (Hannelore Breuer): Expertise in Printing: The New Laser Papers from HP (dated May 13, 2005, available at
h41131.www4.hp.com/Backgrounder_Neue_Laser-Papiere.pdf
(status of 31 Aug. 2010). However, images produced using such paper have a different surface gloss and significantly different haptics in comparison with traditional silver halide images.
To come closer to the goal of a photographic quality, electrophotographically generated images are produced on carrier materials that have the haptics and appearance of a typical silver salt photograph. DE 44 35 350 C1 describes an image-receiving material for electrophotography comprising a base paper coated with thermoplastics and a toner-receiving layer plus an antistatic rear side layer. One disadvantage of this method is that it requires further improvement with regard to toner fixation and the behaviour in the printer. In addition, after an image has been printed, such materials have interfering glossy spots originating from the oily substances which are frequently used as releasing agents in toner formulations.